Bioenergy feedstocks consist primarily of the plant cell wall components cellulose and hemicellulose, and hydrolysis of these polysaccharides to their monomeric sugars involves a set of enzymes acting synergistically to cleave the different chemical linkages (Dodd and Cann, GCB Bioenergy, 1:2, 2009). Although cellulose consists of glucose units linked together in β-1,4-glycosidic linkages, the hemicellulosic component of feedstock may vary in chemical composition. Some feedstock hemicellulose are mainly composed of β-1,4-linked xylose backbones with arabinose side chains, while others are comprised of a larger variety of sugars including galactose and mannose existing as various forms of mannan. Mannans constitute a less significant portion of hemicellulose in bioenergy feedstocks such as switchgrass. However, the presence of mannan may result in linkages that restrict release of fermentable sugars.
A variety of mannans are found in nature. These include linear mannan, glucomannan, galactomannan, and glucogalactomannan. In each case, the polysaccharide contains a β-1,4-linked backbone of mannose residues that may be substituted up to 33% (or up to 50% in hardwoods) with glucose residues (Yeoman et al., Adv Appl Microbiol, Elsivier). In galactomannans or glucogalactomannnans, galactose residues are linked in β-1,6-linkages to the mannan backbone (Moreira and Filho, Appl Microbiol Biotechnol, 79:165, 2008). Therefore, hydrolysis of mannan to its component sugars requires endo-1,4-β-mannanases that hydrolyze the backbone linkages to generate short chain manno-oligosaccharides that are further degraded to monosaccharides by 1,4-β-mannosidases.
Reactions at high temperatures are critical for the enzymatic conversion of plant cell wall polysaccharides to fermentable sugars in the emerging biofuel industry. Thus what is needed in the art are thermostable enzymes for the hydrolysis of mannans contained in biofuel feedstocks. Compositions and methods comprising thermostable endo-β-mannanases will find utility in the enzymatic depolymerization of lignocellulose.